Both natural and synthetic rubber latexes are emulsions of rubber in water. The emulsions are stabilized with proteins or surface-active agents. The median diameter of rubber particles average varies from 0.5 to 2.0 micron. It may be as low as 0.1 micron for some synthetic latexes and tends to be nearly 1 micron for natural latex. The term “latex” is applied to synthetic as well as natural products, and stems from the botanical name of the milky fluid that is derived from rubber trees and which is the source of natural rubber. The term “serum” is used for the aqueous medium.
The process of producing emulsions of rubber in water has been known for many years. For instance, in U.S. Pat. No. 2,595,797 a process with the following steps is disclosed:                1. preparing a solution of a rubber like polymer in a water-insoluble volatile organic solvent in a concentration sufficient for emulsification        2. introducing the solution under pressure into water containing a surface active agent (e.g., dioctyl ester of sodium sulfo-succinic acid)        3. adding an antifoamant (e.g., polysilicone oil) and agitating the mixture until emulsion is obtained        4. removing the solvent by flashing (whilst avoiding excessive foaming)        5. concentrating the solids content of the emulsion by allowing the emulsion to stand for 24 hours and removing the serum (containing less than 2% solids.        
In U.S. Pat. No. 2,799,662 a similar process is described. The method consists of a number of integrated steps which include the dissolving of the dry polymeric material (e.g., rubber) in a suitable selected solvent, dispersing the thus prepared polymer solution into a carefully selected and adjusted water-emulsifier system and, finally, stripping out the solvent to leave the polymer dispersion. According to this reference, in the preparation of the aqueous emulsifier system, it is highly desirable to have two emulsifiers present, one of the type which is hydrocarbon soluble (e.g., alkali metal petroleum sulfonates having 20 to 21 carbon atoms arranged in an alkyl-aryl structure) and one of the water soluble type (e.g., alkali metal sulphate derivatives of higher alcohols). Emulsification of the polymer solvent mixture and the aqueous emulsifier mixture is accomplished under conditions preventing flashing of the solvent.
The problem of emulsion stability when stripping off the solvent is addressed in U.S. Pat. No. 2,871,137, which provides a method for preparing emulsifying agents based on the hydrocarbon polymers that are emulsified.
A method for preparing stable emulsions of polymeric or resinous materials is furthermore described in U.S. Pat. No. 2,947,715. This is accomplished by dissolving the rubber or resin in a suitable solvent, adding a creaming agent to the polymer solution during emulsification, and creaming the resultant latex prior to removal of the solvent, removing the solvent and then again creaming the solvent-free latex.
In U.S. Pat. No. 2,955,094, ortho-phosphoric acid and organic sulphate salts are used as emulsifiers in the preparation of emulsion latexes from hydrocarbon polymers. As indicated in this reference, experience has shown that latexes are relatively unstable and tend to coagulate when subjected to mechanical stress. Mechanical instability may be brought about by the simple movement of an agitator stirring the colloid. Maintenance costs are increased because the equipment becomes coated with the coagulated rubber and furthermore, an appreciable quantity of the rubber is lost. Another type of instability encountered with polymer latexes is that they oil-out and develop coagulum during the solvent stripping step.
U.S. Pat. No. 3,250,737 sets out to produce concentrated latexes of synthetic elastomers from organic solutions thereof in a manner that is both rapid, efficient and economical. This is accomplished by mixing a solution of a synthetic elastomer in an organic solvent, water and an emulsifying agent, homogenizing the mixture at least until the resulting emulsion is stable, stripping the organic solvent at elevated temperatures and pressures below conditions at which water boils, centrifuging the resulting dilute aqueous latex, recovering and recycling the aqueous serum from the said centrifuging step and recovering the concentrated latex. This reference concentrates on the steps of flashing and centrifuging, it is immaterial how the hydrocarbon solution is made.
To give an impression of the overall process as described in the prior art, reference is made to the process in Chapter 9 of the Stanford Research Institute, PEP Report No. 82 of December 1972. Thus, a solution of polyisoprene in isopentane is fed to a premix tank, where it is premixed with a surfactant solution (mostly serum recycle) from the serum storage. The mixture is fed to an emulsification loop in which the recycle to fresh feed ratio is about 2/1. The emulsifier is a high speed (3,500 rpm) centrifugal pump. The emulsion passes to a hold tank where the emulsion is held for 3 hours, permitting any “cream” (emulsion with oversize particles) to rise to the top and be recycled. About 1% emulsion is thus recycled to ensure complete emulsification of any minor amount not previously fully emulsified. If any portion of the emulsified cement is in the form of oversize particles when the solvent is flashed or stripped from this portion, the resulting polymer cannot remain in colloidal suspension, but will deposit out and foul the equipment. From the hold tank, the emulsion is passed to a heater where a substantial portion of the solvent (but only a minor portion of water) is vaporized into gaseous bubbles, causing formation of a foam resembling whipped cream. The foam passes to a time tank to allow the solvent to reach its equilibrium concentration relative to the polymer throughout the foam. The foam is then cooled to 110° F. at about 10 psig, causing the solvent to condense and the foam to collapse. The condensed solvent forms a separate liquid phase from the aqueous emulsion phase. The mixture passes through a coalescer packed with steel wool into a separator. The separated solvent is transferred to the solvent surge tank. The emulsion is centrifuged and concentrated in a centrifuge where a quantity of serum is separated out and recycled to the serum storage tank. Since the polymer particles in the concentrated emulsion still contain solvent, the emulsion is passed through a second stage of foam formation, collapse and phase separation. The second stage separated solvent is also transferred to the solvent surge tank. The second stage emulsion phase is heated to 180° F. in a flash heater to flash off the remaining solvent in a flash tank. This solvent is condensed and stored in the solvent surge tank. Some water is also flashed from the flash tank, and is condensed, separated, and recycled to the surfactant solution tank. The latex from the flash tank contains about 24% rubber solids. It is cooled to 110° F. in an emulsion cooler, concentrated to 64% in a centrifuge and finally collected and stored in a latex product storage vessel. The serum spun out in the concentration step is recycled to the serum storage vessel.
Solvent removal from the emulsion comprising synthetic elastomer, 50-60 wt % solvent, water and emulsifying agent (surfactant) may comprise “weathering”, by which is meant relatively quiescent storage periods under conditions whereby the solvent gradually evaporates. However, in most current applications (surgical gloves and condoms, for instance) the presence of residual solvent is not acceptable. Thus, solvent removal generally comprises flashing operations, distillation, or a foaming operation followed by phase separation of the solvent from the dilute latex. These processes may lead to a reduction in solvent content of 150 ppm or less after removal (calculated on a dilute aqueous rubber emulsion; the solvent content increases when the dilute aqueous emulsion is concentrated). The removal of solvent in the prior art processes is not without problems though.
Each of these operations are generally well understood and broadly disclosed in the art. However, that is not to mean that this step, requiring the removal of a solvent from a multiphase system without destabilizing the emulsion, is not a difficult one. Indeed, this step cannot be compared to the removal of a solvent from a simple two component system (water/solvent). Rather, given the presence of high molecular weight material, surfactant, low boiling hydrocarbon solvents, etc., this step often leads to a significant waste of latex material due to coagulation of the particles of emulsified rubber and fouling of the equipment. This is the reason that the solvent is removed in a multistep process at relatively equal solvent reduction steps. Other problems encountered at this stage are the reduced through-put and/or the increased energy consumption to reduce the solvent to the required low levels. A further problem is the residual level of foam control agents in the final product, which—together with the residual solvent content—should be as low as possible.
It is therefore an object of the present invention to provide an improved process of producing an artificial latex, wherein loss of material and fouling of the equipment in a solvent removal operation conducted in two or more steps is significantly reduced, in a manner that is more efficient in terms of through-put and energy consumption.